U.S. patent number 4,827,217 [Application Number 07/037,030] was granted by the patent office on 1989-05-02 for low noise cryogenic apparatus for making magnetic measurements.
This patent grant is currently assigned to Biomagnetic Technologies, Inc.. Invention is credited to Douglas N. Paulson.
United States Patent |
4,827,217 |
Paulson |
May 2, 1989 |
Low noise cryogenic apparatus for making magnetic measurements
Abstract
Apparatus for performing sensitive magnetic measurements using
cryogenically cooled instrumentation, wherein the instrumentation
is separated from the bubbles present in a cryogenic cooling fluid.
In one embodiment, the magnetic measurement instrumentation is
placed in a tail piece joined by heat conducting bolts to a dewar
containing a cryogenic fluid, and heat from the instrumentation is
conducted to the cryogenic fluid heat sink by metallic strips
reaching to the bolts. The cryogenic fluid does not contact the
instrumentation directly, resulting in a significantly reduced
level of noise in the instrumentation. The tail piece may also be
evacuated to avoid pressure and temperature variations that may
cause noise and affect the magnetic instrumentation.
Inventors: |
Paulson; Douglas N. (Del Mar,
CA) |
Assignee: |
Biomagnetic Technologies, Inc.
(San Diego, CA)
|
Family
ID: |
21892067 |
Appl.
No.: |
07/037,030 |
Filed: |
April 10, 1987 |
Current U.S.
Class: |
324/248; 324/201;
324/225; 335/216; 600/409 |
Current CPC
Class: |
F17C
3/085 (20130101); G01R 33/0358 (20130101); A61B
5/242 (20210101); F17C 2223/0161 (20130101); F17C
2203/0663 (20130101); F17C 2209/228 (20130101); F17C
2270/0509 (20130101); F17C 2221/017 (20130101); F17C
2203/0391 (20130101); F17C 2203/0631 (20130101); F17C
2270/0536 (20130101) |
Current International
Class: |
A61B
5/04 (20060101); F17C 3/08 (20060101); F17C
3/00 (20060101); G01R 33/035 (20060101); G01R
033/035 (); G01R 033/16 (); A61B 005/05 (); G01N
027/72 () |
Field of
Search: |
;324/225,248,346,201,204,224,340 ;128/653 ;335/216 ;336/DIG.1 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
3369174 |
February 1968 |
Groenendyke et al. |
3980076 |
September 1976 |
Wikswo et al. |
4071815 |
January 1978 |
Zemanek |
4437064 |
March 1984 |
Overton, Jr. et al. |
4523147 |
June 1985 |
D'Angelo et al. |
4646025 |
February 1987 |
Martin et al. |
4689559 |
August 1987 |
Hastings et al. |
|
Other References
Brochure of Biomagnetic Technologies Inc., "Introduction to
Biomagnetic Measurements and Instruments", 7 pages, published prior
to 4/10/86. .
D. E. Farrell et al., "Magnetic Measurement of Human Iron Stores,"
IEEE Trans. on Magnetics, vol. MAG-16, Sep. 1980, pp. 818-823.
.
Brittenham et al., "Diagnostic Assessement . . . ", Il Nuovo
Cimento, vol. 2, 1983, pp. 567-581. .
D. E. Farrel et al., "A Clinical System for Accurate Assessment of
Tissue Iron Concentration", Il Nuovo Cimento, vol. 2, 1983, pp.
583-593. .
Wolfgang Ludwig et al., "Eisenuberladung der Leber-eine
Heraus-forderung fur die SQUID-Messtechnik," Postprint of Dornier
GmbH (with English translation), dated Jan. 1986, pp. 22 and
23..
|
Primary Examiner: Strecker; Gerard R.
Attorney, Agent or Firm: Garmong; Gregory O.
Claims
What is claimed is:
1. Cryogenic magnetic measurement apparatus, comprising:
an insulated vessel whose walls are insulated to prevent the
leakage of heat into the interior of said vessel, said vessel
having an opening therein whereby a cryogenic fluid may be
introduced into the interior of said vessel;
an insulated tail piece joined to said insulated vessel and having
walls that are insulated to prevent the leakage of heat into the
interior of said tail piece, said tail piece being constructed of
materials which do not interfere with the taking of magnetic
measurements;
a thermal connector between the interior of said tail piece and the
interior of said insulated vessel, said thermal connector being
heat conducting to conduct heat from the interior of said tail
piece to the interior of said insulated vessel and providing a
liquid-tight seal to prevent cryogenic liquid from leaking from
said insulated vessel into said tail piece;
magnetic measurement apparatus positioned within said tail piece;
and
means for conducting heat from said magnetic measurement apparatus
to said thermal connector, so that in operation heat can be removed
from said measurement apparatus to said thermal connector and
thence to a cryogenic fluid contained within said vessel.
2. The apparatus of claim 1, wherein said thermal connector
comprises metallic bolts extending from the interior said tail
piece to the interior of said insulated vessel.
3. The apparatus of claim 2, wherein said metallic bolts include
copper.
4. The apparatus of claim 1, wherein said means for conducting heat
is a plurality of metal strips connected to said thermal connector
at one end, and to said apparatus at the other.
5. The apparatus of claim 4, wherein said metal strips include
copper.
6. The apparatus of claim 1, wherein said magnetic measurement
apparatus includes a magnetic field sensing coil.
7. The apparatus of claim 1, wherein said magnetic measurement
apparatus includes a superconducting quantum interference
device.
8. The apparatus of claim 1, wherein said magnetic measurement
apparatus includes a magnetization solenoid.
9. Cryogenic magnetic measurement apparatus, comprising:
a vertical insulated dewar insulated from the external
environment;
an insulated, vertical, hollow tail piece insulated from the
external environment, said tail piece being constructed of
materials which do not interfere with the taking of magnetic
measurements and being joined to said dewar but sealed therefrom so
that fluid cannot flow from said dewar into said tail piece;
a plurality of metallic bolts connecting said dewar and said tail
piece, and extending from the interior said tail piece to the
interior of said dewar to conduct heat from said tail piece to said
dewar;
magnetic measurement apparatus including a superconducting magnet,
a magnetic field sensing coil, and a superconducting quantum
interference device positioned within said tail piece; and
a plurality of metal strips connected to said metallic bolts
extending into said tail piece at one end, and to said measurement
apparatus at the other, so that heat can be removed from said
measurement apparatus to said bolts, and thence into said
dewar.
10. The apparatus of claim 9, wherein said magnetic measurement
apparatus further includes a magnetization solenoid.
11. The apparatus of claim 9, wherein said bolts include
copper.
12. The apparatus of claim 9, wherein said metal strips include
copper.
Description
BACKGROUND OF THE INVENTION
This invention relates to cryogenic apparatus, and, more
particularly, to cryogenic apparatus for making sensitive magnetic
measurements.
The human body produces various kinds of energy that may be used to
monitor the status and health of the body. Perhaps the best known
of these types of energy is heat. Most healthy persons have a body
temperature of about 98.6.degree. F. A measured body temperature
that is significantly higher usually indicates the presence of an
infection or other deviation from normal good health. A simple
medical instrument, the clinical thermometer, has long been
available to measure body temperature.
Over 100 years ago, medical researches learned that the body also
produces electrical signals. Doctors today can recognize certain
patterns of electrical signals that are indicative of good health,
and other patterns that indicate disease or abnormality. The best
known types of electrical signals are those from the heart and from
the brain, and instruments have been developed that measure such
signals. The electrocardiograph measures electrical signals
associated with the operation of the heart, and the
electroencephalograph measures the electrical signals associated
with the brain. Such instruments have now become relatively common,
and most hospitals have facilities wherein the electrical signals
from the bodies of patients can be measured to determine certain
types of possible disease or abnormality.
More recently, medical researchers have discovered that the body
produces magnetic fields naturally or when properly stimulated, of
a type completely different from the other types of energy emitted
from the body. The research on correlating magnetic fields and
responses with various states of health, disease and abnormality is
underway. It has been demonstrated, among other things, that
deficiencies or excesses of iron in the body can be determined
quantitatively by the paramagnetic response of iron-containing
molecules in the liver.
The normal, healthy human body typically contains about 60
milligrams of iron per kilogram of body weight (or about four grams
of iron in a typical adult male). A large deficiency or excess of
iron in the body can be clinically significant. A deficiency of
iron deplete bodily reserves, interfere with hemoglobin production,
and lead to anemia in severe cases. An excess of iron can indicate
shifts in body chemistry or disease, such as hereditary
hemochromatosis, in the early stages of refractory anemias and
sometimes in liver disease.
Early diagnosis of iron deficiency or excess imbalances in the body
is particularly important, as these problems can often be
effectively treated at an early stage. Several techniques have been
developed for determining the iron content of the body. Indirect
methods involve measurements of the levels of chemicals whose
presence and amount are thought to be related to iron level in the
body. These methods, such as measurement of serum ferritin or
urinary iron excretion, are not sufficiently quantitative to be
useful in detecting the early stages of an imbalance. Direct
invasive techniques, such as tissue biopsy of the liver, are more
quantitative and accurate, but the discomfort and risks associated
with their use limit their applicability in screening patients for
early indications of iron imbalance.
It has now become possible to make measurements of the iron content
of organs, and particularly the liver where iron reserves are
stored, by direct magnetic measurements that are noninvasive and
therefore particularly suitable for early diagnosis. A biomagnetic
susceptometer is an instrument having a magnetic excitation coil
which excites a paramagnetic response in iron-containing molecules
in the body, and having a very sensitive magnetic detector to
measure the paramagnetic response. The biomagnetic susceptometer is
placed near to the body of the patient, and the patient's iron
levels are measured without any known ill effects on the patient.
The patient is unaware of any sensation of measurement, except that
he is moved cyclically toward and away from the measurement
instrument.
Biomagnetic susceptibility measurements require extraordinarily
sensitive and sophisticated magnetic detectors and techniques for
avoiding spurious noise signals. The fields to be measured from the
liver are typically less than 1/100,000 as great as the magnetic
field of the earth in which the instrument and patient are
immersed. Nearby electrical equipment, metals, implants, and even
the signals from other organs of the body can interfere with the
signal obtained from the organ under study.
At the heart of the biomagnetic susceptometer are specialized
magnetic field sensing coils, and detectors called Superconducting
QUantum Interference Devices (or "SQUIDs"). These devices, which
measure very small magnetic signals, operate in the superconducting
temperature range for their materials of construction. In the
current approach common to most types of superconducting apparatus,
the superconducting temperature is achieved with a bath of a
cryogenic fluid which is maintained as a liquid but boils to remove
heat during operation. The SQUIDS are placed into the cryogenic
fluid bath for stabilized operation at the required
temperature.
In an existing biomagnetic susceptibility measurement instrument,
the magnetic field sensing coils, magnetic excitation coils, and
SQUIDs are immersed in a container of liquid helium at a
temperature of 4.2.degree. K (i.e., near to absolute zero). The
field sensing coils and magnetic excitation coil are placed near
the bottom of the container, within a few centimeters of the
patient. The container, insulation, and related components are made
of special materials that do not interfere with the magnetic
measurements. For example, the container itself is made of a
fiberglass that has substantially no magnetic susceptibility. The
magnetic excitation coil is operated to excite a paramagnetic
response in the iron-containing molecules in the patient's liver,
and the response is detected by the magnetic field sensing coil and
the SQUID working together. The instrumentation is designed to
minimize interference from magnetic signals, both steady and
varying, other than those for which a measurement is sought.
The existing biomagnetic susceptibility instruments have been shown
to give measurements of iron concentration in the liver that
correlate very well with measurements made by biopsy or other
invasive technique, particularly for conditions of excess iron.
However, there is some lack of resolution of the iron content,
particularly for iron levels below normal. In these cases, the
paramagnetic signal may be masked by spurious fields and
influences, and measurement becomes difficult. Complex, expensive
electronics can be used to resolve the small signal for the
background, with reasonable effectiveness. Magnetically quiet
enclosures are also used to reduce the background noise and thence
improve resolution of the paramagnetic response of the liver.
Nevertheless, there continue to be limits to the resolution
possible with existing biomagnetic susceptometers, and it would be
desirable to improve the ability of the instruments to detect weak
signals.
Thus, there exists a need for an improved apparatus for measuring
small biomagnetic responses induced by an external magnetic signal.
Such improved technology would also be of value in other areas
where weak magnetic responses are studied, such as geology and
marine studies. The present invention fulfills this nedd, and
further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an improved cryogenic apparatus for
making measurements of weak magnetic responses and signals, with
greatly reduced noise. With this apparatus, the magnetic noise is
reduced by a factor of over 100 times as compared with existing
approaches, so that magnetic measurements of the human body can be
made with great accuracy and resolution. In particular,
measurements of iron deficiencies of the liver can be made by
non-invasive biomagnetometry techniques.
In accordance with the invention, cryogenic magnetic measurement
apparatus comprises an insulated vessel suitable for containing a
cryogenic fluid to act as a heat sink; an insulated tail piece
having no cryogenic fluid therein, the tail piece being constructed
of materials which do not interfere with the taking of magnetic
measurements; a thermal connector between the interior the tail
piece and the interior of said insulated vessel, the thermal
connector being heat conducting to conduct heat from the interior
of the tail piece to the interior of the insulated vessel and
providing a liquid-tight seal to prevent cryogenic liquid from
leaking from the insulated vessel into the tail piece; magnetic
measurement apparatus positioned within the tail piece; and means
for conducting heat from the magnetic measurement apparatus to the
thermal connector, so that in operation heat can be removed from
the magnetic measurement apparatus into the thermal connector and
thence into the cryogenic liquid heat sink contained with the
apparatus.
It has not previously been appreciated and understood, but has now
been discovered, that a primary source of the remaining noise in
conventional devices for measuring small magnetic signals in large
ambient fields is a physical source, bubbles in the cryogenic fluid
surrounding the field sensing coils and the SQUIDs. The field
sensing coils and SQUIDSs are usually immersed in the cryogenic
fluid, which directly contacts and cools these components of the
magnetic measurement apparatus. The SQUID operates effectively only
in the low, constant temperature possible by this approach. As heat
is transferred into the cryogenic liquid, the fluid boils to
maintain constant temperature and bubbles are formed. These bubbles
rise in the liquid, and the formation and motion of the bubbles,
and the environmental susceptibility variations caused thereby, are
thought to induce spurious magnetic signals which become noise.
The present invention provides a new design of cooler for the
magnetic measurement apparatus. The magnetic measurement apparatus
is not immersed in the cryogenic liquid in this apparatus, and the
cryogenic liquid therefore does not cool the magnetic measurement
apparatus directly. Instead, the magnetic measurement apparatus is
operated within an insulated structure called a tail piece that is
separate from, but attached to, the insulated vessel for holding
the cryogenic liquid, commonly called a dewar. Cryogenic liquid is
not introduced into the tail piece. Means for conducting heat,
preferably a strip of a metallic conductor such as copper, conducts
heat from the magnetic measurement apparatus to a thermal connector
between the tail piece and the insulated vessel holding the
cryogenic liquid. The heat is thereby transferred to the cryogenic
fluid, which boils as it absorbs heat. The cryogenic liquid
therefore acts as a heat sink, rather than direct coolant as in
prior devices.
In the present approach, there is no cryogenic liquid adjacent to,
and bubbling around, the magnetic measurement apparatus in the tail
piece, eliminating this source of magnetic noise. The cryogenic
liquid in the dewar does bubble, but this activity and magnetic
noise are separated sufficiently far from the magnetic measurement
apparatus that there is substantially no interference or noise
resulting from this effect. Although the initial cooling of the
magnetic measurement apparatus by this approach is not as rapid as
with the direct cooling approach, it is still sufficient for the
purposes of the magnetic measurements.
In a preferred embodiment, cryogenic magnetic measurement apparatus
comprises a vertical insulated dewar; an insulated, vertical,
hollow tail piece having no cryogenic fluid therein, the tail piece
being constructed of materials which do not interfere with the
taking of magnetic measurements; a plurality of metallic conductors
connecting the dewar and the tail piece, and extending from the
interior of the tail piece to the interior of the dewar to conduct
heat from the tail piece of the dewar; and magnetic measurement
apparatus including a superconducting magnet, a magnetic field
sensing coil, and a superconducting quantum interference device
positioned within the tail piece.
In a more general aspect of the invention, cryogenic magnetic
measurement apparatus comprises insulated container means for
holding magnetic measurement apparatus that is operable at
cryogenic temperatures; magnetic measurement apparatus contained
within the container means; and means for maintaining the magnetic
measurement apparatus at cryogenic temperature and for avoiding
spurious noise produced by bubbles of vaporized cryogenic fluid. In
this form, the invention extends to a technique for avoiding
interference produced by bubbles of vaporized cryogenic fluid,
based upon the recognition that magnetic susceptibility and
pressure variations caused by the bubbles create significant
magnetic noise. Techniques such as the use of low pressure gas
environments, pumped superfluids, solid cast tail pieces,
electronic nullification of the bubble noise by operating at higher
frequencies, and physical separation of bubbles that may be
produced can be used to effect this result.
In the presently preferred embodiment, the SQUIDs and magnetic
coils are operated in conjunction with a liquid helium bath to
achieve the necessary low operating temperature required for
available superconducting materials. However, as used herein the
term "cryogenic" extends to other fluids that may be used to
maintain reduced, constant operating temperatures. There have been
recent developments in identifying materials having increased
maximum superconducting temperatures. These developments suggest
that in the future there may be magnetic measurement apparatus
operating at liquid nitrogen temperature or even higher
temperatures. It is intended that the present invention not be
limited to its presently preferred form as used in conjunction with
liquid helium cryogenic fluid, but instead cover other cryogenic
coolant fluids such as liquid nitrogen or other liquids that
maintain a low, constant temperature by boiling. The principle of
reduced magnetic noise by avoiding the adverse influence of the
bubbles in the fluid is equally applicable to such other cryogenic
fluids.
It will now be appreciated that the present invention provides an
important advance in the art of magnetic measurement, and
particularly in regard to biomagnetic measurements. The magnetic
noise which interferes with the measurements has been reduced by a
factor of 100 as compared with existing instruments, allowing
better measurements of faint magnetic signals. The apparatus, while
more complex than conventional dewar systems, can be readily
constructed, and easily assembled and disassembled. Other features
and advantages of the present invention will be apparent from the
following more detailed description, taken in conjunction with the
accompanying drawings, which illustrate, by way of example the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevational view of the system for making biomagnetic
susceptibility measurements; and
FIG. 2 is a side sectional view of the apparatus for performing
magnetic measurements.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment of the present invention is utilized in a
biomagnetic susceptometer system 10, illustrated in FIG. 1. An
apparatus 12 for performing magnetic measurements, to be described
in more detail, is supported in a gantry 14, which is sufficiently
wide and tall to permit a patient bed 16 to be rolled under the
apparatus 12. The patient, schematically illustrated at numeral 18,
lies on a vertically movable platform 20 of the bed 16. A
water-filled bag 22, having a water reservoir 24 which provides a
supply of water to the bag 22 to keep it properly full, is
positioned between the body of the patient 18 and the apparatus 12.
A system console 26 is a separate unit having data gathering and
control capabilities. In this system 10, the object of the
measurements, i.e., the body of the patient, is located completely
external to the apparatus 12, as is the case for biomagnetic
measurements. However, the object being magnetically measured could
be located within the apparatus 12, as where the magnetic
characteristics of a small inert object were being studied.
To operate the system 10, a patient 18 is placed onto the bed 16
and moved into place below the apparatus 12, with that portion of
the body of the patient to be studied, typically the liver,
positioned directly below the apparatus 12. The water bag 22 is
positioned between the end of the apparatus 12 and the body of the
patient 18, and filled with water to form a continuous water path
between the apparatus 12 and the patient 18. Biomagnetic
susceptibility measurements are then taken using the apparatus 12.
To null out the effect of the background magnetic environment, the
platform 20 is slowly raised and lowered as the measurements are
taken, as schematically illustrated in FIG. 1. The patient's body,
and the particular organ under study, is thereby moved toward and
away from the apparatus 12. In subsequent analysis of the data
gathered, the organ under study can be viewed as a mass within an
environment of the remainder of the patient's body, and the
magnetic susceptibility of the remainder of the patient's body must
be considered. Water has a magnetic susceptibility similar to that
of the remainder of the body of the patient (excluding the organ
under study), and the placement of the water bag 22 between the
apparatus 12 and the patient 18 reduces the apparent effect of the
remainder of the body moving toward and away from the apparatus.
That is, use of the water bag creates an apparent measurement
environment wherein the organ under study moves in a uniform and
constant water/remainder of the body environment. The contribution
of the organ itself, and excluding the external environment and the
remainder of the patient's body, to the magnetic susceptibility may
therefore be understood directly.
This approach to gathering data on the body is noninvasive--no
breaking of the skin or intrusion by an instrument is required. The
patient need only lie on the bed for the time required to make the
measurements. The susceptibility measurements apply to the body a
small magnetic field about the intensity of a toy bar magnet, which
is not thought to be injurious to the patient in any respect. The
gathering of biomagnetic susceptibility data may be made in less
than one minute, although a large amount of data is ordinarily
taken over a period of about 10-15 minutes to minimize the effect
of transient signals and to obtain a sufficiently large data base
for analysis of the small magnetic signals.
The preferred apparatus 12 of the invention, illustrated in section
in FIG. 2, comprises two major subassemblies, an insulated vessel
30 and a tail piece 31. These two subassemblies 30 and 31 are
joined together in operation, but may be disassembled.
The insulated vessel 30 is similar in construction to a
conventional dewar vessel, but is modified in the manner indicated.
The vessel 30 includes a cylindrical inner wall 32, a cylindrical
middle wall 34, and a cylindrical outer wall 36. The inner wall 32
and the outer wall 36 are constructed of vacuum-tight fiberglass.
The middle wall 34 is constructed of vertically aligned thermal
conductors, preferably aluminum or copper wires, which serve as a
heat conductor to conduct heat upwardly away from the tail piece
31.
The space between the inner wall 32 and the outer wall 36 is
insulated with superinsulation 35, preferably aluminized mylar
film. (In FIG. 2, the superinsulation 35 is illustrated only in one
area for purposes of clarity. However, the superinsulation is found
throughout the space between the walls 32 and 34.) The space
between the inner wall 32 and the outer wall 36 is vacuum tight, so
that a vacuum may be drawn on this space through a port 37. A neck
38 communicating with the interior of the vessel 30 extends
upwardly to the top of the vessel 30. Appropriately sized annular
closures 40, 42, and 44 close the top ends of the inner wall 32,
the middle wall 34, and the outer wall 36, respectively, to the
neck 38. The lower end of the vessel 30 is closed in a manner to be
described subsequently.
In operation, a cryogenic liquid, preferably liquid helium for
presently available superconductor materials, is added to the
interior of the vessel 30 through the neck 38. Instrumentation
leads 46 are also inserted down the neck 38. A series of heat
reflectors 48 in the neck 38 reduce heat loss from the interior of
the vessel 30 to the environment through the neck 38, and the top
of the neck is closed with an sealed closure 50. The middle wall
34, the superinsulation 35, the outer wall 36, the vacuum between
the middle wall 34 and the outer wall 36, the closures 40, 42, and
44, the heat reflectors 48, and the neck closure 50 all aid in
minimizing heat loss from the cryogenic liquid within the vessel
30. A support rod 90 extends vertically down the neck 38 to support
the instrumentation leads 46, heat reflectors 48, and, optionally,
a liquid level detector 92.
The inner wall 32 is closed at the bottom by a circular base plate
52. The base plate 52 is solid, and is sealed to the inner wall 32
by a seal that is not penetrated by the cryogenic liquid. Thus, the
interior wall 32 and the base plate 50 in combination form a
liquid-tight enclosure for retaining the cryogenic liquid. The base
plate 52 is made of a good heat insulator, preferably
fiberglass.
The tail piece 31 is a hollow cylinder having a construction
similar to that of the vessel 30, including an inner wall 54, a
middle wall 55, and an outer wall 56. The outer wall 56 is
constructed of a good insulator material that is vacuum tight and
does not interfere with the magnetic measurements, preferably a
nonmagnetic fiberglass material. The inner wall 54 is constructed
of a vertically extending array of metallic conductor wires,
preferably 0.005 inch diameter copper wires that act as a
radiofrequency interference shield and conduct heat upwardly to the
bottom of the vessel 30.
The space between the middle wall 55 and the outer wall 56 is
continuous with the space between the inner wall 32 and the outer
wall 36 of the vessel 30, and is evacuated through the port 37. The
bottom end of the middle wall 55 is closed with a circular closure
58, and the bottom end of the outer wall 56 is closed with a vacuum
tight seal to a circular closure 60. Both circular closures 58 and
60 are constructed of a material that does not interfere with the
taking of magnetic measurements, preferably nonmagnetic
fiberglass.
The top end of the middle wall 55 is flanged outwardly and the
bottom end of the middle wall 34 of the vessel 30 is flanged
inwardly to provide a structural connection between the two pieces.
The top end of the outer wall 56 and the bottom end of the outer
wall 36 are connected by a vacuum tight annular closure 62, to form
a continuous vacuum tight volume that may be evacuated by pumping
on the vacuum port 37.
Heat is removed from the interior of the tail piece 31 to the
cryogenic fluid heat sink within the vessel 30 through a plurality
of metallic bolts 64, which are preferably constructed of copper or
a copper containing alloy for good thermal conduction. The bolts 64
pass upwardly from the volume within the tail piece 31, through the
base plate 52, and into the interior of the vessel 30. The tops of
the bolts 64 extending into the interior of the vessel 30 contact
the cryogenic liquid contained within the vessel 30, which acts as
a heat sink for the heat conducted from the interior of the tail
piece 31, through the bolts 64, and into the vessel 30.
Heat is conducted for the central volume of the tail piece 31 to
the bolts 64 through a means for conducting heat. Preferably, such
means is a plurality of metal strips 70 or the like, such as copper
strips. The strips 70 are bundled and tied to form a compact,
easily handled structure. Preferably, the strips 70 are a
commercially available product called Litz wire, which is a compact
bundle of about 100 copper wires, each 0.002 inches in diameter.
Heat is conducted from the central volume of the tail piece 31 to
the bolts 64 through the metal strips 70. The heat is then
transferred into the vessel 30 by conduction through the bolts 64.
The heat flows into the cryogenic liquid within the vessel 30,
causing the cryogenic liquid to boil and carry the heat away up the
neck 38.
Magnetic measuremnt apparatus 72 is located within the central
volume of the tail piece 31. A magnetization solenoid 74 (also
sometimes termed a magnetic excitation coil) is wound in a split
pair configuration onto a 1 inch diameter quartz cylinder 76. The
magnetization solenoid 74 typically is formed of about 800 turns of
0.007 inch diameter superconducting niobium-titanium wire, and is
energized to produce a magnetic field of about 50 Gauss at a
distance of 2 centimeters below the bottom of the tail piece 31. A
magnetic field sensing coil 78 is also located within the central
volume, and supported on the quartz cylinder 76. A superconducting
quantum interference device 80, or SQUID, detector is also located
within the central volume, and connected to the magnetic field
sensing coil 78 through a lead 82. Typically, there are provided a
plurality of magnetic field sensing coils 78 and SQUIDs 80 within
the tail piece 31.
In operation of the apparatus 12, a paramagnetic response is
excited in the iron-containing molecules in the organ under study
by the magnetization solenoid 74. The response is received by the
magnetic field sensing coil 78 and detected by the SQUID 80. The
resulting signal is transmitted to the console 26 from the SQUIDs
through instrumentation leads.
It is understood that the removal of heat from the central volume
68 of the tail piece 31 is less efficient by this approach than in
the conventional approach of filling the entire apparatus with the
cryogenic liquid. However, the principal adverse result of the
reduced efficiency is a somewhat longer cool down time at the
beginning of an operating cycle. The greater cool down time is not
a severe problem, inasmuch as the vessel 30 is ordinarily
maintained filled with the cryogenic liquid, and cool down occurs
only infrequently. The cryogenic liquid is drained and the system
brought to ambient temperatures only for repairs or equipment
modification. It is not necessary to drain the system and refill
for each patient.
With a well insulated tail piece 31, the interior is maintained
within about 2.degree. K. of 4.2.degree. K., the temperature of the
preferred liquid helium cryogenic fluid, which is sufficient for
operation of the apparatus 12. As used herein, the term "cryogenic
temperature" refers to a temperature sufficiently low that
superconductivity occurs in the superconducting quantum
interference device. At the present time, such temperatures must be
less than about 10.degree. K. for the devices to operate. A
"cryogenic fluid" or "cryogenic liquid" as used herein is therefore
a liquid that boils at a cryogenic temperature. As the technology
of superconductors advances, new materials are discovered which
permit higher operating temperatures, and in the context of the
present invention much higher temperatures would also be considered
cryogenic temperatures.
The principal advantageous result of the approach of the present
invention is that higher resolution and greater precision can be
obtained in the measured small magnetic signals, because the noise
induced by the boiling cryogenic liquid are removed. In the
preferred embodiment, the magnetic measurement instrumentation is
placed in a volume that does not contact the cryogenic fluid,
although it is cooled by conductors in contact with the cryogenic
fluid. The removal of cryogenic liquid from the vicinity of the
magnetic measurement apparatus avoids nucleation of bubbles
adjacent such apparatus. The bubbles cause low frequency
susceptibility variations adjacent the apparatus as they float
upwardly, which can result in erroneous signals. This source of
error is eliminated in the present apparatus. Placing the apparatus
in a vacumm reduces the possible variation in environmental
pressure which has a similar result. For example, in prior
apparatus the change in barometric pressure above an open dewar
would be sufficient to cause dimensionally induced changes in the
magnetic signal output large enough to overwhelm the signal arising
from the patient. A further result of the present invention is an
increased system time constant, typically about 10 seconds,
yielding improved DC stability.
Thus, it is seen that the approach of the present invention yields
a significant operating improvement over prior designs. The use of
an apparatus which avoids spurious noise produced by bubbles of
vaporized cryogenic fluid results in significantly less magnetic
noise to interfere with the measurements of very weak magnetic
signals. While the invention has been described in relation to a
biomagnetic susceptometer and biomagnetic measurements, its
applicability extends to other types of measurements such as those
in materials science, geology, marine studies, and the like.
Although a particular embodiment of the invention has been
described in detail for purposes of illustration, various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, the invention is not to be
limited except as by the appended claims.
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